Wireless repeaters are commonly used in communication systems to extend the range of a communication system or to provide accessibility to coverage holes within the communication system. The repeater can be configured to receive a transmission at a first frequency and retransmit the transmission at a second frequency. In some embodiments, the repeater can be a single frequency repeater and retransmit signals at the same frequency as the received signal.
The repeater can be configured as a bidirectional repeater. The repeater may receive signals from a local region and retransmit them to a remote receiver in a remote region. The repeater may also receive transmissions from the remote region and retransmit them to a receiver in the local region. The bidirectional repeater may be transparent to a wireless communication transceiver operating in the remote region. That is, the wireless transceiver operating in the remote region may operate in the same manner as if it were communicating directly with a device in the local region, and may not be aware that it is communicating through a repeater.
For some communication systems, the lack of an awareness that a communication link is operating through a repeater does not pose any communication issues. However, in other communication systems, it can be advantageous for a communication device to be aware of the use of a repeater in a communication link.
A Time Division Multiple Access (TDMA) communication system, such as a Global System for Mobile communications (GSM) wireless communication system, active communication devices are allocated receive time slots in which they can expect to receive communications from a base station. Additionally, the communication devices may be assigned a transmit time slot that is relative to the timing of the receive slot. Typically, a communication device can transmit a signal slightly before the assigned transmit time slot to compensate for a propagation delay from the communication device to a receiving base station. However, a communication link that includes a repeater can have additional delays, including the propagation delay from the communication device to the repeater and the delay through the repeater.
Wireless communication systems may also incorporate position location applications. The position location applications can operate based in part on a time of arrival of one or more communication signals. For example, a wireless communication system, such as a GSM system, a Code Division Multiple Access (CDMA) communication system, or a Universal Mobile Telecommunication System (UMTS) that can be based on a GSM system or a CDMA system, can incorporate a position location system where a mobile station can determine its location based in part on a time of arrival of signals from multiple base stations.
If the base station transmission is relayed through a repeater, the mobile station may not be able to determine an accurate time of arrival of the repeated signal. The repeater contributes to a signal delay. Additionally, the aggregate propagation delay from the base station to the repeater and then to the mobile station may not accurately reflect a propagation delay of a signal traveling from the base station directly to the mobile station. Therefore, including the repeater in the communication link creates an inaccuracy in the position location system.
Some communication systems reduce the inaccuracy in a position location system having repeaters by identifying the repeaters as a signal source when the repeater is included as part of the communication link. The repeater can be identified as a signal source, for example by applying a watermark to the signals relayed by the repeater.
The repeater can be configured, for example, to modulate the signals that are relayed. The type of modulation and modulation signal applied by the repeater can identify the particular repeater as the signal source. For example, the repeater can watermark a signal by frequency modulating the signals that are relayed by the repeater. The FM signature used to modulate the signals can identify the particular repeater. In other embodiments, the repeater can be configured to watermark the repeated signals by applying an Amplitude Modulation (AM), Phase Modulation (PM), differential delay modulation, or some other type of modulation to the repeated signals.
Therefore, to extend the range or to provide improved coverage within a wireless communication system, a system operator can selectively add wireless repeaters. The system operator can also configure each repeater to watermark the signals that are repeated to identify the presence and identity of the repeater in the communication link.